1. Field of the Invention
The present invention relates to an evaporated fuel gas adsorbent an evaporated fuel gas trapping apparatus, activated carbon (also termed “active carbon”), and a process for producing the activated carbon. More particularly, the present invention relates to an evaporated fuel gas adsorbent which comprises latent heat storage mediums including containers each of which is made of a flexible film and in each of which a substance that absorbs or releases heat in response to a phase change, relates to an evaporated fuel gas trapping apparatus using the evaporated fuel gas adsorbent, relates to an activated carbon suitable for the evaporated fuel gas adsorbent, that is 1000 to 2500 m2/g in the specific surface area calculated by the BET method (multipoint method), that is 120 cm−1 or less in the half-value width of a D-band peak in the vicinity of 1360 cm−1 in Raman spectroscopic analysis, and that is 100 cm−1 or less in the half-value width of a G-band peak in the vicinity of 1580 cm−1 in Raman spectroscopic analysis, and relates to a process for producing the activated carbon.
2. Description of the Related Art
Activated carbon has excellent adsorbability and hence has been conventionally and widely used to achieve various purposes of use, such as removal of a bad smell, removal of impurities in a liquid, and collection or removal of solvent vapors. In recent years, an issue concerning automobile exhaust emissions and a measure for an improvement in gas mileage have been highlighted, and, from the viewpoint of global environmental protection, various pollution-prevention plans have been carried out for the operations of vehicles. As part of such measures, a porous adsorptive material, such as activated carbon, has been used as an evaporated fuel gas adsorbent. In more detail, a general-purpose vehicle is equipped with an evaporated-fuel-gas processing apparatus in which activated carbon is allowed to adsorb evaporated fuel gas generated from a fuel retaining chamber, such as a fuel tank or a float chamber of a vaporizer, during the traveling and stopping of the vehicle, and then the adsorbed evaporated fuel gas is desorbed by external air drawn thereinto during the traveling of the vehicle, and is sent to an engine intake pipe for combustion treatment.
This evaporated-fuel-gas processing apparatus is called a “canister”, in which a gasoline vapor evaporated from a fuel tank is adsorbed by an adsorptive material, the adsorbed gasoline vapor is then desorbed by external air drawn thereinto during the running of an engine, and the desorbed gasoline vapor is introduced into an engine intake manifold, and is burnt in the engine.
However, the conventional canister has the following essential problems. In detail, the adsorbability of an adsorptive material that adsorbs evaporated fuel gas is improved in proportion to a fall in temperature of the adsorptive material, whereas the desorbability of the evaporated fuel gas from the adsorptive material is increased in proportion to a rise in temperature of the adsorptive material. However, since the adsorption of evaporated fuel gas to an adsorptive material is based on an exothermic reaction, the temperature of the adsorptive material is increased in accordance with the adsorption of the evaporated fuel gas, so that the adsorptivity exhibits a falling tendency. On the other hand, since the desorption of evaporated fuel gas from an adsorptive material is based on an endothermic reaction, the temperature of the adsorptive material is decreased in accordance with the desorption of the evaporated fuel gas, so that the desorptivity exhibits a falling tendency. Thus, if the adsorptive material is used to adsorb evaporated fuel gas in unchanged form, the adsorptivity and desorptivity of the adsorptive material cannot be fully displayed owing to the above-mentioned conflicting actions during adsorption and desorption. This is inefficient.
To improve the performance of a canister, various developments have been advanced up to now. Roughly, two developments can be mentioned, one of which has been advanced while being focused on an adsorptive material superior in the adsorption and desorption of the vapor evaporated from a liquid fuel vapor and the other of which has been advanced while being focused on an apparatus taking consideration of thermal efficiency. In general, an activated carbon, derived from coconut shell carbon, wood-based carbon, or coal and the like, or a shaped activated carbon produced by molding activated carbon is used as the adsorptive material. For example, a shaped activated carbon produced by adding short fibers to granular activated carbon and then molding the resulting mixture together with an emulsion serving as a binder is known as an adsorptive material (see Patent document 1: Japanese Published Examined Patent Application No. S48-7194).
An activated carbon modified by oxidation treatment is also known, and is described as being excellent especially for a vehicular canister using a mixed vapor of gasoline and alcohol (see Patent Document 2: Japanese Published Examined Patent Application No. H1-52324). Additionally, it is known to use two kinds of activated carbons differing in average packing density as an activated carbon superior in the adsorption of evaporated fuel gas and in manufacturing costs (see Patent Document 3: Japanese Published Examined Utility Model Application No. H5-17411).
A shaped activated carbon improved in mechanical strength and in abrasive resistance is also known. The following are examples thereof: i.e., a shaped activated carbon produced by pulverizing lignocellulose-based granular activated carbon, then mixing the resulting powder with bentonite clay, then extruding the resulting mixture, then dehydrating and drying the resulting pellets, and subjecting the pellets to thermal treatment (see Patent Document 4: Japanese Published Unexamined Patent Application No. H9-249409); a shaped activated carbon produced by pulverizing lignocellulose-based granular activated carbon, then mixing the resulting powder with an organic binder, then extruding the resulting mixture, then dehydrating and drying the resulting pellets, and subjecting the pellets to thermal treatment (see Patent Document 5: Japanese Published Unexamined Patent Application No. H10-203811); and a molded activated carbon specified by a butane working capacity and an abrasion rate (see Patent Document 6: Japanese Published Unexamined Patent Application No. 2000-313611).
In recent years, canisters have been required to be reduced in size and weight. To meet this requirement there is a need to optimize pores of an adsorptive material so as to improve the performance per volume. When the conventional techniques are seen from this viewpoint, the activated carbon or the shaped activated carbon described in Patent Documents 1 to 3 mentioned above cannot fully satisfy the requirement.
Each of Patent Documents 4 and 5 mentioned above discloses a technique concerning the shaped activated carbon produced by pulverizing wood-based activated carbon then extruding the resulting powder while adding a binder thereto, and calcining the resulting pellets. The thus-produced shaped activated carbon can be improved in mechanical strength and in abrasive resistance, but has difficulty in showing its sufficient performance because the pores are closed with the binder. The same applies to the molded activated carbon disclosed by Patent Document 6.
Activated carbon is also known which has been developed paying attention to the pore size distribution of the activated carbon in order to efficiently trap evaporated fuel. As examples thereof, the following are known: i.e., a fuel-evaporation preventing medium that consists of fibrous activated carbon having a specific pore distribution (see Patent Document 7: Japanese Published Examined Patent Application No. S61-55611); a method for improving the pore distribution of activated carbon by adjusting the concentration of oxygen in a heat treatment process (see Patent Document 8: Japanese Published Unexamined Patent Application No. H6-127912); and a granular evaporated-fuel adsorbing agent that has pores of 50% by weight or more within the range of a pore diameter of 1.4 to 2.8 nm and that has a pore volume of 0.3 mL or more per milliliter (mL) of the adsorbing agent (see Patent Document 9: Japanese Published Unexamined Patent Application No. 2003-314387).
The fibrous activated carbon disclosed by Patent Document 7 has only a small number of useless pores and can be regarded as effective in preventing the evaporation of fuel from the viewpoint of contact efficiency. The fibrous activated carbon shows a high performance per weight, but does not necessarily have a satisfactory performance per volume. Additionally, the fibrous activated carbon is expensive, and has difficulty in industrial applicability.
Patent Document 8 proposes a method for improving pore distribution by setting the oxygen concentration during a 200 to 400° C. calcining process at 5% by volume or more and setting the oxygen concentration during a more-than-400° C. calcining process at less than 5% by volume. However, since the reaction between carbon and oxygen is an exothermic reaction, the temperature partially runs away, so that a burning reaction is easily caused. On the other hand, if the temperature is low, the reaction does not easily proceed, and temperature control cannot be easily performed, thus making stable production difficult.
The evaporated-fuel adsorbing agent disclosed by Patent Document 9 is proposed as a substitute for fibrous activated carbon with the aim of withstanding long-term use. However, in detail, the evaporated-fuel adsorbing agent disclosed thereby is obtained by using the effect of expensive graphite powder, and is hardly adequate from the viewpoint of industrial applicability, for example, because the activation speed is unsatisfactory. Additionally, although an adsorptive material using a coal is described as favorable, no disclosure is made about what kind of coal is suitable as the coal.
On the other hand, to solve these problems from the viewpoint of thermal efficiency, there is a method for controlling temperature by flowing a medium, such as water, from the outside. However, if such a medium is flowed from the outside, much time is consumed to control the temperature, which includes that of the inside of the adsorptive material, because the thermal conductivity of the adsorptive material is low although temperature control can be easily performed near the medium. Additionally, equipment used to flow such a medium and a driving utility are required.
An evaporated fuel trapping apparatus is also known in which a solid heat storage material that is greater in specific heat than activated carbon is dispersed into the activated carbon. Metallic materials, various ceramic materials, glass, or inorganic materials are used as the solid heat storage material (see Patent Document 10: Japanese Published Unexamined Patent Application No. S64-36962). However, since the evaporated fuel trapping apparatus disclosed by Patent Document 10 uses sensible heat, a thermal disadvantage arises in comparison with a heat quantity needed to improve adsorption and desorption, and a large amount of solid heat storage materials must be mixed therewith to produce a desired effect. As a result, the ratio of the activated carbon is relatively lowered, and, disadvantageously, a total amount of adsorption is not improved even if the problem of temperature caused during adsorption and desorption is solved.
A latent-heat storage type adsorbent is also known which is composed of an adsorptive material and a heat storage medium including microcapsules in each of which a substance that absorbs or releases latent heat in response to temperature change is encased. This adsorbent is used for a canister (see Patent Document 11: International Publication WO 03/106833 A1). This adsorbent can prevent both a performance decrease caused by the heat incoming and outgoing in accordance with adsorption and desorption, i.e. prevent a temperature rise caused by heat generation during adsorption and a temperature fall caused by heat absorption during desorption. Therefore, presumably, this adsorbent is useful in improving the performance of a canister in which heat comes and goes in response to adsorption and desorption.
The adsorbent including the microcapsules disclosed by Patent Document 11 uses a substance that absorbs or releases latent heat in response to temperature change as a heat storage material, and hence an advantageous effect is expected to be achieved by mixing a small amount of heat storage material. However, even if the adsorptive material and a liquid in which the microcapsules have been dispersed are equally mixed together and are dried, practical problems will arise. For example, pores of the adsorptive material will be closed when the adsorbent is used, thereby lowering its adsorptivity. Additionally, for example, vibrations will cause a separation between the microcapsule encasing the heat storage material and the adsorptive material, thereby making it impossible to fulfill its proper heat-absorbing-and-generating capabilities.
Patent Document 11 also proposes a method for mixing an adsorptive material and microcapsules in each of which a powdery heat storage material is encased together and then compressing the resulting mixture into molded pieces. This method seems to be effective from the viewpoint of heat transfer efficiency, because the heat storage material and the adsorptive material come into close contact with each other. However, the process of compressing the mixture into molded pieces, which is a complex process, is needed, and there is a fear that the microcapsule will be destroyed during the compressing process, and the phase-change substance will leak out therefrom. Therefore, it is necessary to lower molding pressure and perform molding so as not to destroy the microcapsule. As a result, although the problem of temperature caused during adsorption and desorption is solved, the amount of activated carbon per unit volume is decreased, and hence the total amount of adsorption remains without being increased.